Metal nanoparticle-decorated nanotubes for gas sensing

ABSTRACT

Disclosed herein are methods of producing metal nanoparticle-decorated carbon nanotubes. The methods include forming a reaction mixture by combining a first solution with a second solution, wherein the first solution comprises polymer-coated metal nanoparticles comprising metallic nanoparticles coated with a polymer, and wherein the second solution comprises carbon nanotubes. The methods also include heating the reaction mixture to a temperature greater than a glass transition temperature of the polymer for a time sufficient to cause the polymer-coated metal nanoparticles to bind to the carbon nanotubes forming the metal nanoparticle-decorated carbon nanotubes.

RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 16/996,239, filed Aug. 18, 2020, entitled “METALNANOPARTICLE-DECORATED NANOTUBES FOR GAS SENSING”, which is a divisionalof U.S. patent application Ser. No. 15/582,172, filed Apr. 28, 2017,entitled “METAL NANOPARTICLE-DECORATED NANOTUBES FOR GAS SENSING,” whichissued on Nov. 10, 2020 as U.S. Pat. No. 10,830,721, the entire contentsof which are incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under Contract No.DE-AR0000542 MONITOR-SPHINCS awarded by the Advanced Research ProjectsAgency-Energy. The Government has certain rights in this invention.

TECHNICAL FIELD

The presently disclosed embodiments are directed to gas sensors, and,more particularly, nanoparticle-based materials for gas sensors.

BACKGROUND

Various techniques are utilized for gas leak detection, with eachapproach having advantages trade-offs. Such techniques include, forexample, catalytic bead sensors, metal-oxide-semiconductor (MOS)sensors, non-dispersive infrared sensors, and infrared laser-basedsensors.

Catalytic bead and MOS sensors are compact and can easily be integratedinto printed circuit boards. They also have high methane detectionlimits, which are sufficient for improving safety, but insufficient fordetecting low level leaks that, when undetected, can add up and have anadverse environmental effect. Moreover, these sensors consume enoughpower to not be compatible with long-term battery powered operation, andsuffer from interference from similar gases since they utilize chemicalinteractions to perform their measurements.

Infrared-based sensors are highly sensitive and generally immune tointerference, but are often expensive, bulky, and have high powerrequirement.

SUMMARY

The following presents a simplified summary of various aspects of thepresent disclosure in order to provide a basic understanding of suchaspects. This summary is not an extensive overview of the disclosure. Itis intended to neither identify key or critical elements of thedisclosure, nor delineate any scope of the particular embodiments of thedisclosure or any scope of the claims. Its sole purpose is to presentsome concepts of the disclosure in a simplified form as a prelude to themore detailed description that is presented later.

In one aspect of the present disclosure, a composition comprises: carbonnanotubes having an average degree of functionalization with carboxylicacid groups and/or hydroxyl groups that is less than 3 percent by weight(wt %) based on a total weight of the carbon nanotubes; andpolymer-coated metal nanoparticles bound to the carbon nanotubes.

In one embodiment, the polymer-coated metal nanoparticles arenon-covalently bound to the carbon nanotubes.

In one embodiment, the carbon nanotubes are substantially free ofcarboxylic acid functional groups and hydroxyl functional groups.

In one embodiment, the carbon nanotubes comprise single-walled carbonnanotubes or multi-wall carbon nanotubes.

In one embodiment, the polymer-coated metal nanoparticles each comprisea metallic core and a polymer layer covalently bound to the metalliccore. In one embodiment, the polymer layer comprises a hydrophobicpolymer. In one embodiment, the metallic core comprises a metal selectedfrom a group consisting of palladium, iridium, rhodium, platinum, andgold. In one embodiment, the polymer layer comprisespoly(vinylpyrrolidinone), and wherein the metallic core comprisespalladium.

In one embodiment, the composition is dispersed in an organic solvent.

In another aspect of the present disclosure, a sensor for detecting gascomprises: an electrode assembly comprising electrodes; and agas-adsorbing material disposed between the electrodes of the electrodeassembly. In one embodiment, the gas-adsorbing material comprises:carbon nanotubes; and polymer-coated metal nanoparticles bound to thecarbon nanotubes.

In one embodiment, the electrode assembly is operatively coupled to aprocessing device, wherein the processing device is to measure changesin resistivity of the gas-adsorbing material that result from gasmolecules adsorbed to the gas-adsorbing material. In one embodiment, thesensor has detection limit of 100 parts per million (ppm) duringoperation in an ambient environment having a relative humidity from 0%to 80%. In one embodiment, the sensor is adapted to selectively detectmethane.

In one embodiment, the polymer-coated metal nanoparticles arenon-covalently bound to the carbon nanotubes.

In one embodiment, an average degree of functionalization of the carbonnanotubes with carboxylic acid groups and/or hydroxyl groups is lessthan 3 wt % based on a total weight of the carbon nanotubes.

In another aspect of the present disclosure, a method of producing metalnanoparticle-decorated carbon nanotubes comprises: forming a reactionmixture by combining a first solution with a second solution, whereinthe first solution comprises polymer-coated metal nanoparticlescomprising metallic nanoparticles coated with a polymer, and wherein thesecond solution comprises carbon nanotubes; and heating the reactionmixture to a temperature greater than a glass transition temperature ofthe polymer for a time sufficient to cause the polymer-coated metalnanoparticles to bind to the carbon nanotubes forming the metalnanoparticle-decorated carbon nanotubes.

In one embodiment, the polymer-coated metal nanoparticles arefully-formed prior to forming the reaction mixture.

In one embodiment, the polymer-coated metal nanoparticles arenon-covalently bound to the carbon nanotubes.

In one embodiment, an average degree of functionalization of the carbonnanotubes with carboxylic acid groups and/or hydroxyl groups is lessthan 3 wt % based on a total weight of the carbon nanotubes.

In one embodiment, the method further comprises dispersing the metalnanoparticle-decorated carbon nanotubes in a non-aqueous solvent-basedink.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings, in which:

FIG. 1 is an illustration of an embodiment of a polymer-coated metalnanoparticle;

FIG. 2 illustrates a reaction process for producing polymer-coated metalnanoparticles according to one embodiment of the present disclosure;

FIG. 3 is an illustration of carbon nanotubes with increasing amounts ofcarboxylic acid functionalization;

FIG. 4 is a flow diagram illustrating a method for producing a metalnanoparticle-decorated nanotube in accordance with embodiments of thepresent disclosure;

FIG. 5 is a schematic illustrating an exemplary metalnanoparticle-decorated nanotube produced in accordance with embodimentsof the present disclosure;

FIG. 6 is a flow diagram illustrating a method for fabricating a sensorin accordance with embodiments of the present disclosure;

FIG. 7A is a schematic illustrating a top down view of an exemplarysensor chip according to an embodiment of the present disclosure;

FIG. 7B is a schematic illustrating a cross-sectional view of the sensorchip;

FIG. 7C is a micrograph showing an ink containing metalnanoparticle-decorated nanotubes that was printed onto interdigitatedelectrodes of an exemplary sensor;

FIG. 7D is a schematic illustrating the sensor chip operatively coupledto a processing device according to an embodiment of the presentdisclosure;

FIG. 8 is an energy-dispersive x-ray spectrum of metalnanoparticle-decorated nanotubes.

DETAILED DESCRIPTION

Described herein are embodiments of metal nanoparticle-decoratednanotubes, methods for producing the same, and sensors incorporating thesame. Certain embodiments relate to a gas (e.g., methane) sensor thatuses a hydrophobic composition comprising carbon nanotubes (e.g.,single-walled carbon nanotubes, or “SWCNTs”) decorated with metalnanoparticles as a sensing material. The sensing material (also referredto herein as a “gas-adsorbing material”) may be placed in betweeninterdigitated electrodes of a sensor. When methane gas adsorbs to thesensing material, its electronic state is changed resulting in a changein resistivity that is proportional to the amount of methane adsorbed.This change in resistivity can be measured via a number of resistivitymeasurement techniques (e.g., voltammetry). The sensor, when adapted formethane sensing, can be regenerated by flushing with methane-free gas.

Current approaches to detect methane focus on creating extremely compactand low power devices. These approaches focus on the adsorptive effectsof different types of materials. This includes catalytic metal-decoratedcarbon nanotubes, thin films utilizing catalytic metals, and graphene orcarbon nanotube with chemical modifications. However, suchphysisorption-based methane sensors lose their detection capability inthe presence of water vapor to the point that many cannot detect evenpure methane mixed with water vapor. Since humidity is presenteverywhere in the atmosphere, current methane sensors based onadsorption effects cannot work reliably in practical situations.

Without being bound by theory, it is proposed herein that a reason forthis performance loss is due to the accumulation of water ontoadsorptive surfaces by interaction with the surface carboxylic acidgroups or hydroxyl groups through hydrogen bonding. It is hypothesizedthat the interaction between methane and the metal nanoparticles ishydrophobic, and would be independent of the water vapor concentrationif the presence of hydrophilic functional groups is reduced orsuppressed entirely. As described herein, an approach to limit theimpact of water vapor includes suppressing the available carboxylic acid(“—COOH”) sites for water vapor to adsorb to. This includes, forexample, capping the —COOH groups with hydrophobic metal nanoparticles,as well using polymers to bind the nanoparticles to the nanotube (e.g.,through non-covalent interactions).

In one embodiment, organic polymer-coated palladium nanoparticles(produced separately from the SWCNTs) are mixed with the SWCNTs andheated to a temperature above the glass transition temperature of thepolymer. On cooling, it was found that palladium-decorated SWCNTs wereobtained in the cases of highly —COOH-functionalized SWCNTs, SWCNTs withlow —COOH functionalization, and even SWCNTs without any —COOHfunctionalization. This result is unexpected in view of previousapproaches, which suggest the need for highly functionalized —COOH inorder to grow Pd nanoparticles onto SWCNTs. Without being bound bytheory, it is hypothesized that an additional mechanism by which the Pdparticles bind onto the surfaces of the SWCNTs is due to heat-drivensoftening of the coating polymer present onto the Pd nanoparticles,allowing the Pd nanoparticles to be bound non-specifically andnon-covalently to the SWCNTs.

The embodiments of the present disclosure provide low-power, low-costalternatives to traditional methane sensing techniques that are capableof detecting leaks in virtually any environment, even those with high orvariable humidity. Moreover, the embodiments of the present disclosureafford several advantages over traditional gas-sensing approaches. Theembodiments utilizing printable metal nanoparticle-decorated nanotubesare capable of reliably detecting low concentrations of methane leaks inthe presence of variable humidity levels. The embodiments are adaptablefor use in portable or hand-held devices, or as part of a network ofsensors in natural gas extraction fields. The embodiments can also beused for sensing other gases, for example, by replacing palladium withother metals such as, but not limited to, iridium, rhodium, platinum, orgold. In addition, the disclosed methods are modularized and easilyscalable for mass production.

FIG. 1 is an illustration of an embodiment of a polymer-coated metalnanoparticle 100. The polymer-coated metal nanoparticle 100 includes ametallic core 102 and a polymer layer 104 that coats the metallic core102. In some embodiments, the metallic core 102 may be crystalline,polycrystalline, amorphous, or a combination thereof. In someembodiments, the metallic core 102 may comprise a single metal species(e.g., a transition metal), or an alloy of different metal species. Insome embodiments, the metal is selected from palladium, iridium,rhodium, platinum, and gold. For example, in specific embodiments forsensing methane, the metallic core 102 may be a palladium core. Incertain embodiments, a diameter of the metallic core 102 may range, forexample, from 1 nm to 100 nm, from 1 nm to 50 nm, from 1 nm to 20 nm,from 1 nm to 10 nm, or from 1 nm to 5 nm.

The polymer layer 104, as illustrated, comprises a plurality of linearpolymers bound to a surface of the metallic core 102. In someembodiments, branched polymers may be used. In some embodiments, thepolymers may be covalently bound to the metallic core 102. In otherembodiments, the polymers may be physically adsorbed to the metalliccore 102, with the adsorption being driven by, for example, hydrophobicinteractions. The polymers may be densely packed on the surface of themetallic core 102 such that steric forces between the polymers cause thepolymers to extend from the surface. The rigidity of the polymer layermay be a function of polymer chemical structure, packing density,surface curvature, polymer molecular weight and polydispersity, andsolvent conditions. The polymer layer 104 may serve to mitigateinter-particle forces between the polymer-coated metal nanoparticle 100and other polymer-coated metal nanoparticles to prevent aggregationwhile dispersed in solvent. In some embodiments, the polymer layer 104comprises polymers of a single type or of different types. In someembodiments, the polymer layer 104 comprises hydrophobic polymers. Insome embodiments, the polymer layer 104 comprises polyvinylpyrrolidone(PVP).

FIG. 2 illustrates a reaction process for producing polymer-coated metalnanoparticles according to one embodiment of the present disclosure.Specifically, FIG. 2 illustrates a reaction process for producingPVP-coated palladium nanoparticles, where insoluble PdCl₂ is firstreacted with HCl to form soluble H₂PdCl₄, which is then subsequentlyreacted with ethylene glycol in the presence of PVP to produce Pd(0)metal nanoparticles.

In certain embodiments, metal nanoparticle-decorated nanotubes areproduced by a process utilizing a separate step for synthesizing themetal nanoparticles and a separate step for synthesizing nanotubes,which affords greater control over the end product than methods thatgrow the nanoparticles directly at functionalized locations on thenanotube surface.

As used herein, “nanotube” refers to a hollow structure having nanoscaledimensions along at least two axes. Nanotubes may be cylindrical inshape and have high aspect ratios, with diameters from, for example, 5nanometers (nm) to 100 nm, and lengths that generally range from 20 nmto 1 micrometer (μm) or greater. Although single wall carbon nanotubesare described herein in exemplary embodiments, the embodiment s extendto other types of carbon nanotubes such as multi wall carbon nanotubes.The electronic properties of carbon nanotubes can vary from metallic tosemiconducting as a function of diameter and chirality (which describesa degree of “twisting” in the positions of atoms along a length of thenanotube).

FIG. 3 is an illustration of carbon nanotubes with increasing amounts ofcarboxylic acid functionalization. Carbon nanotube 300 represents acarbon nanotube with no functionalization or minimal functionalizationwith —COOH or other hydrophilic functional groups. Carbon nanotube 310represents a carbon nanotube with a low or moderate degree of —COOHfunctionalization, and carbon nanotube 320 represents a carbon nanotubewith a high degree of —COOH functionalization. The —COOH may be presentalong the surface of the carbon nanotubes, as well as at the ends of thecarbon nanotubes.

FIG. 4 is a flow diagram illustrating a method 400 for producing metalnanoparticle-decorated nanotubes in accordance with embodiments of thepresent disclosure. The method 400 begins at block 402, where a firstsolution comprising polymer-coated metal nanoparticles is provided. Thepolymer-coated metal nanoparticles may have structures represented bythe polymer-coated metal nanoparticle 100 described with respect to FIG.1 . The polymer-coated metal nanoparticles may be synthesized asdescribed herein (e.g., Example 4 below), or using any adaptations orother suitable synthesis methods as would be appreciated by one ofordinary skill in the art. In some embodiments, each polymer-coatedmetal nanoparticle comprises a palladium core and a polymer layercomprising PVP. In some embodiments, the polymer-coated metalnanoparticles are dispersed in an organic solvent.

At block 404, a second solution comprising carbon nanotubes is provided.The carbon nanotubes may be unfunctionalized, may be substantially freeof carboxylic acid (—COOH) and/or hydroxyl (—OH) functionalization(e.g., below a detectable limit), or may have an average degree of —COOHand/or —OH functionalization less than 3% percent by weight (wt %) basedon a total mass of the carbon nanotubes (i.e., including any suchfunctionalization). In some embodiments, the carbon nanotubes may havean average degree of —COOH and/or —OH functionalization greater than orequal to 3 wt %. The wt % of —COOH and —OH functionalization, asdiscussed herein, is measured by the weight loss in thermogravimetricanalysis (TGA) at temperatures below 300° C. when the measurement is runin Ultra High Purity (UHP) grade inert gas. The carbon nanotubes may besynthesized as described herein (e.g., Examples 1-3 below), or using anyadaptations or other suitable synthesis methods as would be appreciatedby one of ordinary skill in the art. In some embodiments, the carbonnanotubes are SWCNTs. In some embodiments, the carbon nanotubes aredispersed in an aqueous solvent.

At block 406, a reaction mixture is formed by combining the firstsolution with the second solution. At block 408, the reaction mixture isheated to a temperature greater than a glass transition temperature ofthe polymer of the polymer-coated metal nanoparticles (a glasstransition temperature for PVP, for example, may vary from 100° C. to180° C. depending on its molecular weight). In some embodiments, thetemperature is greater than but within 50° C. of the glass transitiontemperature. In some embodiments the temperature is greater than butwithin 25° C. of the glass transition temperature. In some embodimentsthe temperature is greater than but within 10° C. of the glasstransition temperature. In some embodiments, the temperature is from120° C. to 180° C. In some embodiments, the temperature is from 120° C.to 140° C. In some embodiments, the temperature is from 150° C. to 180°C. In some embodiments, the temperature is maintained for a timeduration of 30 minutes to 4 hours. In some embodiments, the temperaturemay be varied from one temperature (e.g., 120° C. to 140° C.) for afirst time duration (e.g., 30 minutes to 4 hours) to a secondtemperature (150° C. to 180° C.) for a second time duration (e.g., 30minutes to 4 hours). In some embodiments, the nanoparticle-coated metalnanoparticles are treated with a solvent to remove solvent-accessiblePVP from surfaces of the nanoparticles. FIG. 5 illustrates an exemplarymetal nanoparticle-decorated nanotube 500 having polymer-coated metalnanoparticles 504 bound to a carbon nanotube 502. The polymer-coatedmetal nanoparticles 504 are bound to the surface of the carbon nanotube502 non-covalently by the adhesive nature of the polymer.

In certain embodiments, a sensor material for sensing methane gas may beproduced (e.g., in accordance with the method 400) by immobilizingcoated palladium nanoparticles on surfaces of SWCNTs with little to no—COOH functionalization, dispersing the resulting metalnanoparticle-decorated nanotubes in a non-aqueous solvent-based ink, andprinting the ink onto a sensor chip.

FIG. 6 is a flow diagram illustrating a method 600 for fabricating asensor in accordance with embodiments of the present disclosure. Themethod 600 begins at block 602, where an ink comprising metalnanoparticle-decorated nanotubes is provided. The metalnanoparticle-decorated nanotubes may correspond to any metalnanoparticle-decorated nanotubes described herein (e.g., PVP-coatedpalladium nanoparticles bound to SWCNTs).

At block 604, a sensor chip or substrate having an electrode arrayformed thereon is provided.

Reference is now made to FIGS. 7A and 7B, which are schematicsillustrating top down and cross-sectional views, respectively, of anexemplary sensor chip 700 according to an embodiment of the presentdisclosure. The sensor chip 700 includes a substrate 702 having sensors704A-704D formed thereon. Although four sensors are depicted, it isnoted that any suitable number of sensors may be used. The substrate 702comprises a non-conductive material, such as polyethylene naphthalate(PEN), polyimide, or any other suitable non-conductive material. In someembodiments, a thickness of the substrate may be selected to facilitatea secure connection to a type of zero insertion force (ZIF) connector(e.g., substrate thickness of 250 μm).

The sensors 704A-704D may comprise any suitable electrode material suchas copper, graphite, titanium, silver, gold, platinum, or combinationsthereof. The sensors 704A-704D may be shaped to facilitate electricalcontact with external components for sensor readout. A single counterelectrode 706 may also be formed on the substrate 702, which may beshared by each of the sensors 704A-704D to reduce the total number ofelectrodes on the sensor chip 700.

The sensors 704A-704D and the common electrode 706 may together define aregion with interdigitated electrodes 708. FIG. 7B illustrates across-section through the interdigitated electrodes 708 of the sensor704C (as indicated by the dotted line in FIG. 7A), which shows an activeregion 710 defined between the interdigitated electrodes 708 where agas-sensing material may be deposited. In certain embodiments, athickness of the interdigitated electrodes 708 may range from 100 nm to1 μm. A pitch 710 between adjacent interdigitated electrodes 708 mayrange, for example, from 50 μm to 5 mm.

The sensor chip 700 may be designed such that a portion of the sensorchip 700 can be directly inserted into an electrical connector forresistance measurement. In order to achieve desired resistance levels ofthe printed substance, the sensor chip 700 can be designed to vary thenumber, duty cycle, and dimensions (including thickness) ofinterdigitated electrodes 708, as well as the gap distance betweenadjacent electrodes. The dimensions of the printed leads of the sensors704A-704D may be chosen such that the lead resistances for the commonelectrode path and the sensor path are nearly in order to cancel outtheir influence in the resistance measurements. The substrate 702 may bedesigned for ratiometric 3-wire resistance measurements, but may also becompatible with traditional 3-wire resistance measurements and 2-wireresistance measurements.

Referring once again to FIG. 6 , at block 606, the ink is depositedbetween electrodes of the electrode array, and the solvent of the ink isallowed to evaporate leaving a gas-sensing material comprising metalnanoparticle-decorated nanotubes. In certain embodiments, the ink isdeposited by printing directly (e.g., using inkjet printing) on thesensors. In some embodiments, the ink is deposited using other suitablemethods, such as pipetting, spin-coating, or dip-coating. FIG. 7C is amicrograph showing the ink printed onto interdigitated electrodes of anexemplary sensor.

FIG. 7D illustrates the operation of the sensor chip 700, which isillustrated as being operatively coupled to a processing device 750(e.g., via a plurality of ZIF pins 754). The processing device mayinclude one or more electronic components, such as a MUX 752, that maybe configured to measure the resistivity of the gas sensing materialusing the various on-chip sensors. In some embodiments, the processingdevice 750 represents one or more general-purpose processing devicessuch as a microprocessor, central processing unit, or the like. Forexample, the processing device 750 may be a complex instruction setcomputing (CISC) microprocessor, reduced instruction set computing(RISC) microprocessor, very long instruction word (VLIW) microprocessor,or a processor implementing other instruction sets or processorsimplementing a combination of instruction sets. The processing device750 may also be one or more special-purpose processing devices such asan application specific integrated circuit (ASIC), a field programmablegate array (FPGA), a digital signal processor (DSP), network processor,or the like. The processing device 750 may be configured to performvarious operations, such as applying electrical signals, measuringelectrical signals, and computing data based on the measured electricalsignals. In certain embodiments, the processing device 750 may be asingle device or a device that controls other devices. For example, theprocessing device 750 may be configured to perform resistivitymeasurements, or may control one or more other devices that performresistivity measurements.

In some embodiments, a sensor produced according to the method 600 mayoperate over a wide relative humidity range in ambient conditions. Asused herein, “ambient conditions” refers to the conditions of a typicallaboratory environment with a temperature of 20±5° C. and a pressure of1±0.1 atmospheres (ATM). In certain embodiments, during operation of thesensor in an ambient environment having a relative humidity from 0% to80%, the sensor has a methane detection limit of 100 ppm. In someembodiments, the sensor may have a lower detection limit, e.g., 50 ppmor 10 ppm. The sensor may achieve such performance over a temperaturerange outside of the ambient conditions (e.g., from −5° to 50° C.).

ILLUSTRATIVE EXAMPLES

The following examples are set forth to assist in understanding theinvention and should not, of course, be construed as specificallylimiting the invention described and claimed herein. Such variations ofthe invention, including the substitution of all equivalents now knownor later developed, which would be within the purview of those skilledin the art, and changes in formulation or minor changes in experimentaldesign, are to be considered to fall within the scope of the inventionincorporated herein.

Example 1: Fabrication of SWCNTs with High Degree of —COOHFunctionalization

A suspension was produced by dispersing 200 mg of SWCNTs (NanoAmor) in50 mL of deionized water and sonicating for 2 hours (20 W). Thesuspension was then placed in a round bottom flask and 50 mL ofconcentrated HNO₃ (16 M) were added. The reaction volume was refluxedunder magnetic stirring at 120° C. for 5 days, and then cooled down toroom temperature. The reaction volume was washed with deionized (DI)water and centrifuged to separate the particles. The washing andcentrifugation process was repeated until neutral pH was reached.

Example 2: Fabrication of SWCNTs with Low Degree of —COOHFunctionalization

The process from Example 1 was repeated except that after the reactionvolume was heated and stirred for 2 hours instead of 5 days.

Example 3: Fabrication of SWCNTs with Negligible —COOH Functional Groups

A suspension was produced by dispersing 200 mg of SWCNTs (NanoAmor) in100 mL of concentrated hydrochloric acid and bath sonicated for 30minutes at room temperature. The suspension was diluted with 2 L water(deionized water filtered under vacuum using a 0.2 μmpolytetrafluoroethylene membrane) and washed with deionized water untilneutral pH was reached. The filtered material was then dispersed in 40mL deionized water for later use.

Example 4: Fabrication of Polymer-Coated Metal Nanoparticles

Polymer-coated palladium (Pd) nanoparticles were synthesized using thepolyol reduction method: a reaction volume was prepared in a Schleckflask by mixing 1.5 mL of 65 mM H₂PdCl₄ with 5 mL of 5 wt %polyvinylpyrrolidone (PVP) in ethylene glycol (EG) solution and adding100 mL of EG. The reaction volume was saturated with argon followed bydegassing in vacuum 5 times to remove dissolved oxygen. The PVP-coatedPd nanoparticles were then produced by heating the degassed solution at130° C. for 3 h followed by 160° C. for 1 hour while stirring at 500rpm. FIG. 7A is an electron micrograph showing the resulting PVP-coatedPd nanoparticles.

Example 5: Fabrication of Polymer-Coated Metal Nanoparticle-DecoratedSWCNTs

The Pd nanoparticle solution of Example 4 was cooled to room temperatureafter which 20 mL of SWCNTs dispersed in water was added to the Pdnanoparticle solution and degassed, followed by heating the degassedsolution at 130° C. for 3 h and heating at 160° C. for 1 h whilestirring. Excess acetone was added to the Pd-SWCNT solution andcentrifuged at 4000 rpm for 10 min, and the supernatant solution wasdiscarded to remove Pd-decorated SWCNTs (Pd-SWNTs). The particles werefurther washed with excess ethanol under stirring for 15 minutesfollowed by centrifugation at 4000 rpm for 10 minutes to remove the PVPfrom the Pd-SWCNT particles. The washing sequence was repeated fivetimes.

FIG. 8 is an energy-dispersive x-ray (EDX) spectrum of the Pd-SWCNTparticles.

For simplicity of explanation, the methods of this disclosure aredepicted and described as a series of acts. However, acts in accordancewith this disclosure can occur in various orders and/or concurrently,and with other acts not presented and described herein. Furthermore, notall illustrated acts may be required to implement the methods inaccordance with the disclosed subject matter.

Although embodiments of the disclosure were discussed in the context ofcomposite materials (e.g., nanoparticle-nanotube composite materials)and devices utilizing the same for gas sensing, one or more of thecomponents described herein may be adapted for use in other devices andsystems. Thus, embodiments of the disclosure are not limited to gassensors and the specific constituents described.

In the foregoing description, numerous details were set forth. It willbe apparent, however, to one of ordinary skill in the art having thebenefit of this disclosure, that the embodiments of the presentdisclosure may be practiced without these specific details. In someinstances, certain structures and devices are shown in block diagramform, rather than in detail, in order to avoid obscuring the presentdisclosure. It is to be understood that the details of such structuresand devices, as well as various processes for producing the same, wouldbe within the purview of one of ordinary skill in the art.

The terms “above,” “under,” “between,” and “on” as used herein refer toa relative position of one layer with respect to other layers. As such,for example, one layer deposited or disposed above or under anotherlayer may be directly in contact with the other layer or may have one ormore intervening layers. Moreover, one layer deposited or disposedbetween layers may be directly in contact with the layers or may haveone or more intervening layers. In contrast, a first layer “on” ordeposited “onto” a second layer is in contact with that second layer.

The words “example” or “exemplary” are used herein to mean serving as anexample, instance, or illustration. Any aspect or design describedherein as “example” or “exemplary” is not necessarily to be construed aspreferred or advantageous over other aspects or designs. Rather, use ofthe words “example” or “exemplary” is intended to present concepts in aconcrete fashion. As used in this application, the term “or” is intendedto mean an inclusive “or” rather than an exclusive “or”. That is, unlessspecified otherwise, or clear from context, “X includes A or B” isintended to mean any of the natural inclusive permutations. That is, ifX includes A; X includes B; or X includes both A and B, then “X includesA or B” is satisfied under any of the foregoing instances. In addition,the articles “a” and “an” as used in this application and the appendedclaims should generally be construed to mean “one or more” unlessspecified otherwise or clear from context to be directed to a singularform. Reference throughout this specification to “an embodiment” or “oneembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearances of the phrase “anembodiment” or “one embodiment” in various places throughout thisspecification are not necessarily all referring to the same embodiment.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments of andmodifications to the present disclosure pertaining to nanocompositematerials and uses thereof, in addition to those described herein, willbe apparent to those of ordinary skill in the art from the precedingdescription and accompanying drawings. Thus, such other embodiments andmodifications thereto are intended to fall within the scope of thepresent disclosure. Further, although the present disclosure has beendescribed herein in the context of a particular embodiment in aparticular environment for a particular purpose, those of ordinary skillin the art will recognize that its usefulness is not limited thereto andthat the present disclosure may be beneficially implemented in anynumber of environments for any number of purposes. Accordingly, theclaims set forth below should be construed in view of the full breadthand spirit of the present disclosure as described herein, along with thefull scope of equivalents to which such claims are entitled.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

What is claimed is:
 1. A method of producing metalnanoparticle-decorated carbon nanotubes, the method comprising: forminga reaction mixture by combining a first solution with a second solution,wherein the first solution comprises polymer-coated metal nanoparticlescomprising metallic nanoparticles coated with a polymer, and wherein thesecond solution comprises carbon nanotubes; and heating the reactionmixture to a temperature greater than a glass transition temperature ofthe polymer for a time sufficient to cause the polymer-coated metalnanoparticles to bind to the carbon nanotubes forming the metalnanoparticle-decorated carbon nanotubes.
 2. The method of claim 1,wherein the polymer-coated metal nanoparticles are fully-formed prior toforming the reaction mixture.
 3. The method of claim 2, wherein thepolymer-coated metal nanoparticles are non-covalently bound to thecarbon nanotubes.
 4. The method of claim 1, wherein an average degree offunctionalization of the carbon nanotubes with carboxylic acid groupsand/or hydroxyl groups is less than 3 wt % based on a total weight ofthe carbon nanotubes.
 5. The method of claim 1, further comprising:dispersing the metal nanoparticle-decorated carbon nanotubes in anon-aqueous solvent-based ink.
 6. The method of claim 1, wherein thepolymer-coated-metal nanoparticles are coated with a hydrophobic polymerlayer.
 7. The method of claim 1, wherein the carbon nanotubes aresubstantially free of carboxylic acid functional groups and hydroxylfunctional groups.
 8. A method of producing metal nanoparticle-decoratedcarbon nanotubes, the method comprising: forming polymer-coated metalnanoparticles by dispersing a polymer and a salt precursor of a metalinto a solvent, reducing the metal salt precursor of the metal with areducing reagent; forming a reaction mixture by combining a firstsolution with a second solution, wherein the first solution comprisespolymer-coated metal nanoparticles comprising metallic nanoparticlescoated with a polymer, and wherein the second solution comprises carbonnanotubes; and heating the reaction mixture to a temperature greaterthan a glass transition temperature of the polymer for a time sufficientto cause the polymer-coated metal nanoparticles to bind to the carbonnanotubes forming the metal nanoparticle-decorated carbon nanotubes. 9.The method of claim 8, wherein an average degree of functionalization ofthe carbon nanotubes with carboxylic acid groups and/or hydroxyl groupsis less than 3 wt % based on a total weight of the carbon nanotubes. 10.The method of claim 8, wherein the solvent is both a solvent and areducing reagent.
 11. The method of claim 9, wherein the solvent and areducing reagent is ethylene glycol.
 12. The method of claim 8, furtherinvolving heating of the solution of polymer and a salt precursor of ametal into the solvent.
 13. The method of claim 8, further involvingremoval of excess of un-bound polymer onto the polymer-coatednanoparticles by washing with a first solvent, separation andredispersion of the polymer-coated nanoparticles into a second solventto form a polymer-coated nanoparticle dispersion that is free of unboundpolymer, (i.e., does not contain unbound polymer).
 14. The method ofclaim 8, wherein the carbon nanotubes are selected from a group ofsingle wall and multi-wall carbon nanotubes.
 15. The method of claim 8,wherein the diameter of the metallic core of the polymer coated metalnanoparticles decorated carbon nanotubes is comprised in a range from 1nm to 20 nm.
 16. The method of claim 8, wherein the diameter of themetallic core of the polymer coated metal nanoparticles decorated carbonnanotubes is comprised in a range from 1 nm to 20 nm and the carbonnanotubes are single wall carbon nanotubes.
 17. A method of producingmetal alloy nanoparticle-decorated carbon nanotubes, the methodcomprising: forming polymer-coated metal nanoparticles by dispersing apolymer and a mixture of two or more salt precursors of metals into asolvent, reducing the mixture of metal salt precursors with a reducingreagent forming polymer coated alloy metal nanoparticles containing twoor more metals; forming a reaction mixture by combining a first solutionwith a second solution, wherein the first solution comprisespolymer-coated metal nanoparticles comprising metallic nanoparticlescoated with a polymer, and wherein the second solution comprises carbonnanotubes; and heating the reaction mixture to a temperature greaterthan a glass transition temperature of the polymer for a time sufficientto cause the polymer-coated metal nanoparticles to bind to the carbonnanotubes forming the metal nanoparticle-decorated carbon nanotubes. 18.The method of claim 17, wherein the salt precursors of thepolymer-coated metal alloy nanoparticles is selected from a group ofmetal ions that after reduction process form metals selected from agroup of palladium, iridium, rhodium, platinum, and gold.
 19. The methodof claim 17, wherein an average degree of functionalization of thecarbon nanotubes with carboxylic acid groups and/or hydroxyl groups isless than 3 wt % based on a total weight of the carbon nanotubes.